Why does SPF fail?

What the SPF number actually tells you

SPF measures protection against UVB, the ultraviolet rays primarily responsible for sunburn. UVA contributes to premature skin aging and skin cancer. Choose broad spectrum protection to cover both, rather than treating the largest number on the bottle as its entire CV. Use SPF 30 or higher for daily outdoor exposure; for extended time outdoors, choose water resistant SPF 50 or higher. Higher SPF offers more UVB protection, but does not last longer or remove the need to reapply.345

How to avoid application gaps

1. 1Apply generously to dry skin before going outside. Allow at least 15 minutes and follow the bottle's directions. For prolonged exposure with a risk of burning, NHS advice is to apply 30 minutes before leaving and again just before going out.
2. 2Use enough to cover every exposed area. Most adults need at least about one ounce for skin not covered by clothing, with more needed for a larger body. For the face, the American Academy of Dermatology advises at least one teaspoon. A thin layer gives less protection.
3. 3Cover the easy omissions: ears, neck, backs of hands, tops of feet and the hairline. Protect an exposed scalp with sunscreen or a hat. Use an SPF 30 or higher lip product for the lips.
4. 4For more thorough coverage, apply a first layer, rub it in, then immediately apply a second layer using the same amount. The double coat method helps address underapplication.
5. 5Reapply about every two hours while outdoors, and immediately after swimming or sweating. Reapply after towel drying or whenever sunscreen has rubbed off.

Water resistant does not mean waterproof

US water resistance labels specify 40 or 80 minutes of tested protection during swimming or sweating. Check that duration before getting in the water; the usual two hour interval is not permission to ignore it. Water can wash sunscreen away, and its cooling effect can hide the feeling of getting burned. Water, sand and snow also reflect UV rays, increasing exposure.256

Check the bottle, not just your application

Expired sunscreen no longer provides its advertised protection. Discard a bottle past its expiry date. If there is no printed date, discard it three years after purchase; if you cannot remember how old it is, replace it. Store sunscreen somewhere cool and dark, not in a hot car. Heat and sunlight can make it degrade faster.127

What if you still tan or develop dark spots?

A tan is a response to potentially damaging UV exposure, not proof of safe sun exposure. Visible sunlight can also worsen dark spots, particularly in darker skin tones. For that concern, choose a tinted broad spectrum SPF 30 or higher sunscreen containing iron oxide. Clouds still let UV through, so a grey sky is not a substitute for sunscreen, however convincingly it ruins the beach photos. Wear protective clothing and sunglasses, seek shade, and do not extend your time in the sun because you applied sunscreen.256

Who needs extra care, and when to get help

Some medicines increase sun sensitivity; ask your health care professional about precautions for yours. Take extra care if you burn easily, have many moles or have a personal or family history of skin cancer. All skin tones need protection. Keep babies under six months out of direct sunlight. If you burn, cool the skin with cool water and stay out of the sun until redness has gone. Seek medical help if you feel unwell or develop severe swelling or blisters.26